QtCore: fix GCC 7 warnings

GCC 7 warns about implicit fall-throughs now.

Fix by adding Q_FALLTHROUGH.

Change-Id: I482ab4c6adc469b11e1fd163516ff486b3b55ef7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/statemachine/qstatemachine.cpp

@@ -2024,7 +2024,9 @@ void QStateMachinePrivate::processEvents(EventProcessingMode processingMode)
         if (QThread::currentThread() == q->thread()) {
             _q_process();
             break;
-        } // fallthrough -- processing must be done in the machine thread
+        }
+        // processing must be done in the machine thread, so:
+        Q_FALLTHROUGH();
     case QueuedProcessing:
         processingScheduled = true;
         QMetaObject::invokeMethod(q, "_q_process", Qt::QueuedConnection);

src/corelib/tools/qdatetimeparser.cpp

@@ -1139,6 +1139,7 @@ end:
                             }
                             break; }
                         }
+                        Q_FALLTHROUGH();
                     case MonthSection:
                         if (sn.count >= 3) {
                             const int currentMonth = newCurrentValue.date().month();